Nematodes (derived from the Greek word for thread) are active, flexible, elongate, organisms that live on moist surfaces or in liquid environments, including films of water within soil and moist tissues within other organisms. While only 20,000 species of nematode have been identified, it is estimated that 40,000 to 10 million actually exist. Many species of nematodes have evolved to be very successful parasites of plants and animals and are responsible for significant economic losses in agriculture and livestock and for morbidity and mortality in humans (Whitehead (1998) Plant Nematode Control. CAB International, New York).
Nematode parasites of plants can infest all parts of plants, including roots, developing flower buds, leaves, and stems. Plant parasites are classified on the basis of their feeding habits into the broad categories migratory ectoparasites, migratory endoparasites, and sedentary endoparasites. Sedentary endoparasites, which include the root knot nematodes (Meloidogyne) and cyst nematodes (Globodera and Heterodera) induce feeding sites and establish long-term infections within roots that are often very damaging to crops (Whitehead, supra). It is estimated that parasitic nematodes cost the horticulture and agriculture industries in excess of $78 billion worldwide a year, based on an estimated average 12% annual loss spread across all major crops. For example, it is estimated that nematodes cause soybean losses of approximately $3.2 billion annually worldwide (Barker et al. (1994) Plant and Soil Nematodes: Societal Impact and Focus for the Future. The Committee on National Needs and Priorities in Nematology. Cooperative State Research Service, US Department of Agriculture and Society of Nematologists). Several factors make the need for safe and effective nematode controls urgent. Continuing population growth, famines, and environmental degradation have heightened concern for the sustainability of agriculture, and new government regulations may prevent or severely restrict the use of many available agricultural anthelmintic agents.
There are a very small array of chemicals available to effectively control nematodes (Becker (1999) Agricultural Research Magazine 47(3):22-24; U.S. Pat. No. 6,048,714). In general, chemical nematicides are highly toxic compounds known to cause substantial environmental damage and are increasingly restricted in the amounts and locations in which they can be used. For example, the soil fumigant methyl bromide which has been used effectively to reduce nematode infestations in a variety of specialty crops, is regulated under the U.N. Montreal Protocol as an ozone-depleting substance and is undergoing phase out in the US and world wide (Carter (2001) California Agriculture, 55(3):2). It is expected that strawberry and other commodity crop industries will be significantly impacted if a suitable replacement for methyl bromide is not found. Similarly, broad-spectrum nematicides such as Telone (various formulations of 1,3-dichloropropene) have significant restrictions on their use because of toxicological concerns (Carter (2001) California Agriculture, 55(3):12-18). Organophosphate and carbamate pesticides are another important class of nematicides undergoing regulatory review and several of these compounds are currently being phase out (e.g., fenamiphos, terbufos, cadusafos).
To date little success has been achieved in finding safe effective replacements for the toxic but efficacious conventional nematicides. A recent example of the poor efficacy of many newer potential replacements for organophosphates and carbamates is the study of alternatives to fenamiphos for management of plant parasitic nematodes in bermudagrass. In these trials, none of the experimental treatments reduced population densities of the plant parasitic nematodes, or consistently promoted turf visual performance or turf root production (Crow (2005) Journal of Nematology, 37(4):477-482). Consequently there remains an urgent need to develop environmentally safe, efficacious methods of controlling plant parasitic nematodes
Some plant species are known to be highly resistant to nematodes. The best documented of these include marigolds (Tagetes spp.), rattlebox (Crotalaria spectabilis), chrysanthemums (Chrysanthemum spp.), castor bean (Ricinus communis), margosa (Azardiracta indica), and many members of the family Asteraceae (family Compositae) (Hackney & Dickerson. (1975) J Nematol 7(1):84-90). In the case of the Asteraceae, the photodynamic compound alpha-terthienyl has been shown to account for the strong nematicidal activity of the roots. Castor beans are plowed under as a green manure before a seed crop is set. However, a significant drawback of the castor plant is that the seed contains toxic compounds (such as ricin) that can kill humans, pets, and livestock and is also highly allergenic. In most cases however, the active principle(s) for plant nematicidal activity has not been discovered and it remains difficult to derive commercially successful nematicidal products from these resistant plants or to transfer the resistance to crops of agronomical importance such as soybeans and cotton.
Genetic resistance to certain nematodes is available in some commercial cultivars (e.g., soybeans), but these are restricted in number and the availability of cultivars with both desirable agronomic features and resistance is limited. Furthermore, the production of nematode resistant commercial varieties by conventional plant breeding based on genetic recombination through sexual crosses is a slow process and is often further hampered by a lack of appropriate germplasm.
Chemical means of controlling plant parasitic nematodes continue to be essential for many crops which lack adequate natural resistance or a source of transgenic resistance. In the specialty markets, economic hardship resulting from nematode infestation is particularly high in strawberries, bananas, and other high value vegetables and fruits. In the high-acreage crop markets, nematode damage is greatest in soybeans and cotton. There are however, dozens of additional crops that suffer from significant nematode infestation including potato, pepper, onion, citrus, coffee, sugarcane, greenhouse ornamentals and golf course turf grasses.
To be useful in modern agriculture nematicides must have high potency, a broad spectrum of activity against different strains of nematodes and should not be toxic to non-target organisms.
Nematode parasites of vertebrates (e.g., humans, livestock and companion animals) include gut roundworms, hookworms, pinworms, whipworms, and filarial worms. They can be transmitted in a variety of ways, including by water contamination, skin penetration, biting insects, or by ingestion of contaminated food.
In domesticated animals, nematode control or “de-worming” is essential to the economic viability of livestock producers and is a necessary part of veterinary care of companion animals. Parasitic nematodes cause mortality in animals (e.g., heartworm in dogs and cats) and morbidity as a result of the parasites' inhibiting the ability of the infected animal to absorb nutrients. The parasite-induced nutrient deficiency leads to disease and stunted growth in livestock and companion animals. For instance, in cattle and dairy herds, a single untreated infection with the brown stomach worm can permanently restrict an animal's ability to convert feed into muscle mass or milk.
Two factors contribute to the need for novel anthelmintics and vaccines to control animal parasitic nematodes. First, some of the more prevalent species of parasitic nematodes of livestock are building resistance to the anthelmintic drugs available currently, meaning that these products are losing their efficacy. These developments are not surprising because few effective anthelmintic drugs are available and most have been used continuously. Some parasitic species have developed resistance to most of the anthelmintics (Geents et al. (1997) Parasitology Today 13:149-151; Prichard (1994) Veterinary Parasitology 54:259-268). The fact that many of the anthelmintic drugs have similar modes of action complicates matters, as the loss of sensitivity of the parasite to one drug is often accompanied by side resistance—that is, resistance to other drugs in the same class (Sangster & Gill (1999) Parasitology Today 15(4):141-146). Secondly, there are some issues with toxicity for the major compounds currently available.
Infections by parasitic nematode worms also result in substantial human mortality and morbidity, especially in tropical regions of Africa, Asia, and the Americas. The World Health Organization estimates 2.9 billion people are infected, and in some areas, 85% of the population carries worms. While mortality is rare in proportion to infections, morbidity is substantial and rivals diabetes and lung cancer in worldwide disability adjusted life year (DALY) measurements.
Examples of human parasitic nematodes include hookworms, filarial worms, and pinworms. Hookworms (1.3 billion infections) are the major cause of anemia in millions of children, resulting in growth retardation and impaired cognitive development. Filarial worms invade the lymphatics, resulting in permanently swollen and deformed limbs (elephantiasis), and the eyes, causing African river blindness. The large gut roundworm Ascaris lumbricoides infects more than one billion people worldwide and causes malnutrition and obstructive bowel disease. In developed countries, pinworms are common and often transmitted through children in daycare.
Even in asymptomatic parasitic infections, nematodes can still deprive the host of valuable nutrients and increase the ability of other organisms to establish secondary infections. In some cases, infections can cause debilitating illnesses and can result in anemia, diarrhea, dehydration, loss of appetite, or death.
Despite some advances in drug availability and public health infrastructure and the near elimination of one tropical nematode (the water-borne Guinea worm), most nematode diseases have remained intractable problems. Treatment of hookworm diseases with anthelmintic drugs, for instance, has not provided adequate control in regions of high incidence because rapid re-infection occurs after treatment. In fact, over the last 50 years, while nematode infection rates have fallen in the United States, Europe, and Japan, the overall number of infections worldwide has kept pace with the growing world population. Large scale initiatives by regional governments, the World Health Organization, foundations, and pharmaceutical companies are now underway attempting to control nematode infections with currently available tools, including three programs for control of Onchocerciasis (river blindness) in Africa and the Americas using ivermectin and vector control; The Global Alliance to Eliminate Lymphatic Filariasis using DEC, albendazole, and ivermectin; and the highly successful Guinea Worm Eradication Program. Until safe and effective vaccines are discovered to prevent parasitic nematode infections, anthelmintic drugs will continue to be used to control and treat nematode parasitic infections in both humans and domestic animals.
Certain insecticidal oxazoles (U.S. Pat. No. 4,791,124) and thiazoles (U.S. Pat. No. 4,908,357) and nematicidal pyrazoles (U.S. Pat. No. 6,310,049) have been disclosed in the art. The present invention discloses other oxazoles, oxadiazoles and thiadiazoles with surprisingly potent nematicidal activity showing activity comparable to commercial standards. Commercial level nematicidal potency has not previously been demonstrated with oxazoles, oxadiazoles and thiadiazoles. Importantly, these compounds are broadly active against nematodes yet safe to non-target organisms.
U.S. Pat. No. 4,791,124 disclosed certain oxazoles and thiazoles with nematicidal activity against Meloidogyne incognita (root knot nematode) at 10 parts per million. However, compounds were not titrated to lower doses and not shown to have potency comparable to commercial standards.
U.S. Pat. No. 6,310,049 disclosed certain nematicidal pyrazoles with activity against root knot nematode. Several pyrazole compounds are shown having activity at 100 ppm in an in vitro assay with a small subset of the compounds having activity at 50 ppm in a soil based greenhouse. One compound is disclosed as having greenhouse activity at 20 ppm and a single compound as having greenhouse activity at 5 ppm. It is not clear if any of these compounds have potency comparable to commercial standards.
Some oxadiazoles compounds having substituted furan or thiophene rings but not unsubstituted furan or thiophene rings are disclosed as being apoptosis inducers and useful as chemotherapeutic against certain cancers (Zhang et al. 2005 J Med. Chem. 48(16):5215-23). Notwithstanding some superficial chemical similarities the nematicidal analogs of this invention do not induce apoptosis in mammalian cells and have equal potency against wild type C. elegans nematodes and ced-3 or ced-4 C. elegans mutants deficient in apoptosis. These analogs are therefore structurally and functionally distinct from the apoptosis inducing oxadiazoles disclosed by Cai et al in U.S. Pat. No. 7,041,685.